Why does Mary tell her children not to be ashamed of being smarter than the local kids? Why was this also a bad move on her part?

Mary tells the principal that her children are gifted. The principal, unable to understand Jeannette's accent: he thinks they have learning disabilities and places them in a remedial class for slower children. Word gets out that the Walls children are supposed to be "gifted" and they are made fun of by other students.